This invention relates to methods of and circuits for suppressing clutter and noise for automatic target detection radars. Radar clutter signals are unwanted echo signals caused, e.g., by rain, land, or sea. Clutter signal amplitude depends on radar range and direction, and is statistically distributed over a wide range of amplitudes as a function of radar range and azimuth. Noise signals, on the other hand, are unwanted signals having random fluctuations also with a wide range of amplitudes. Common sources of noise include internally generated receiver noise and jamming interference from external sources. Unlike clutter echo, noise at the receiver output occurs at all radar ranges.
Automatic detection radars use an interference thresholding circuit called CFAR (constant false alarm rate). A CFAR establishes threshold levels at a plurality of radar resolution cells to automatically reject clutter and noise (see, e.g., M. I. Skolnik, Introduction to Radar Systems, 3rd Edition, McGraw-Hill, 2001, pp. 295-298). Then, a signal larger than the threshold is assumed to be due to a target and one below the threshold is assumed to be caused by either the noise and/or clutter. The most commonly used CFAR is the range CFAR. A range CFAR sets a threshold level at each range cell based on sampling the strength of a received signal in neighboring range cells. To accomplish this, the CFAR obtains an aggregate (usually an average) of the strengths of the radar signal sampled at neighboring range cells, and based on the aggregate""s magnitude it sets a threshold. Then for each range cell, a target signal is provided as output if its magnitude exceeds the threshold at said each range cell; otherwise the CFAR output is zero.
Experience shows that the target detection capability of CFAR processing is degraded when it is required to operate in scenarios that require rejecting a variety of clutter types and noise. This is because the amplitude statistics of noise and clutter are dramatically different, and the statistics of clutter differ significantly with differences in the physical features of rain, land and sea. The present invention provides an effective way to maintain a sensitive target detection capability and yet effectively control false alarms caused by noise and clutter. This is done whether targets are in the presence of noise, clutter, or a mixture of the two.
The input to a CFAR is supplied from a unipolar detector output. Traditionally, the amplitude of received signals has been obtained with crystal diode detectors or demodulators. Also, the I, Q detector is commonly used, especially in MTI and pulse Doppler radars. The amplitude of a received echo signal which is input to a radar receiver that provides I and Q signals is proportional to (I2+Q2)1/2, where I and Q are the in-phase and quadrature signals (see, e.g., M. 1. Skolnik, Introduction to Radar Systems, 3rd Edition, McGraw-Hill, 2001, pp. 288-290). To minimize requirements on processor dynamic range, some radars use just the I and Q magnitudes to approximate (I2+Q2)1/2, thereby avoiding the increased dynamic range caused by squaring. The present invention is applicable to improving detection performance and suppressing noise and clutter when using any of the above mentioned detectors or any other detector that provides a unipolar signal.
The invention is applicable to improving the detection sensitivity of automatic target detection radar and to the automatic suppression of noise and clutter. It improves detection and interference suppression by adaptively establishing a threshold for detecting targets and for rejecting noise and clutter, based on two factors: (1) the mean or another aggregate strength of interfering noise and clutter located within resolution cells near a target""s resolution cell, and (2) the statistical distribution of said noise and clutter. As explained in material that follows, the invention employs an amplitude expandor that functions in operative association with a CFAR device. As a consequence, the CFAR threshold level is adaptively established to maximize target detection sensitivity by establishing a desired threshold level consistent with both the strength of and the statistical characteristics of the interfering noise and clutter.
The objects of the invention are to provide circuits and methods that:
(1) Will improve the target detection sensitivity and the capabilities for suppressing noise and clutter
(2) Will provide an improved method for automatic detection of targets in the presence of noise and clutter, comprising a signal expandor that functions in operative association with a CFAR device
(3) Will provide a noise and clutter threshold that responds to the strength of noise and clutter and to the statistical nature of said interference, and to respond to said strength and statistical nature independently of one another.
(4) Will provide an improved automatic target detection and interference suppression capability through use of a relatively simple expandor device that can be readily implemented with digital processing technology and which can function in operative association with commonly used CFAR technology.
This invention will process unipolar radar signals, and including signals obtained with traditional crystal diode detectors or demodulators, and with I, Q detectors that are used in MTI and pulse doppler radar. Therefore the objects of the invention are also to provide circuits and methods that:
(5) Can be used with a MTI or a pulse Doppler radar that uses MTI cancellers, and
(6) Can be used with a MTI or a pulse Doppler radar using a plurality of doppler filters.
To accomplish these objects, the invention includes an amplitude expandor means that functions in operative association with a CFAR device. Use of an amplitude expandor for expanding the amplitude range of the signals before being input to a CFAR emphasizes the stronger amplitude components of the interference. In this way, the mean of the noise and/or clutter interference at the CFAR is raised differently depending on the statistics of the interference. Thus, the more spiky clutter (higher percentage of strong echoes), which requires a higher threshold relative to its average signal strength for rejecting its spikes (peaks in signal strength), inherently creates the needed adaptive rejection mechanism. Therefore the expandor/CFAR combination is an efficient method for detecting targets, and yet it can effectively reject the prevailing noise and/or clutter. In other words, because of the different statistical features of noise and clutter, the expandor/CFAR combination adapts to the target and interference environment simultaneously by two independent processes: the average strength and the statistical characteristics of nearby interference.